Is creating a nonce token for custom meta_box redundant?

Hello.
Just as explained at Codex page for a function “add_meta_box”, I added custom meta_box for a certain custom post type , like the following(using php5.3);

add_action("add_meta_boxes_somepost_type", function(){
  add_meta_box("HooHoo","hoo", function($post){
    //render hidden input field for nonce
    wp_nonce_field("HooHoo","hoo");//<-----(*)

    //render some <input> or <select>elements...

    //(abbr.)

  );
};
add_action("save_post", function($post_id, $post){
  //verify nonce
  if(!wp_verify_nonce($POST["hoo"],"HooHoo")return;

  //and, verify anything else...

  add_post_meta($post_id, "hoo", sanitize_text_field($_POST["hoodata"]),true);
},10,2);

Also the example code shown at Codex Page, a callback for “add_meta_box” creates a special nonce token (at the above code(*)) and an action hooked into “save_post” verifies the token. But a default nonce token “_wpnonce” is also created and is verified before executing save_post action by a function “check_admin_referer” and consequently verification for CSRF is nealy completed before verifying the special nonce created at meta_box. I think it redundant to create and check a nonce token peculiar for a certain plugin.
Anyone knows reasons for creating such a nonce at meta_box?